Image forming materials using a photosensitive material such as a photopolymer and methods of forming images thereby are known, and some of them have so far been utilized in the preparation of a color proof for color correction in printing. For example, there is known a method of forming images by the steps of preparing an image forming material by coating on a support a photosensitive composition containing an addition-polymerizable or cross-linkable compound and a photopolymerization initiator, and if necessary, a colorant and a binder comprising an organic high polymer, exposing the photosensitive composition layer to harden its exposed portion, and dissolving an unexposed portion to remove it.
However, conventional methods which employ a liquid developer have problems concerning environmental protection and safety for workers, in addition to a disadvantage of requiring a large apparatus.
On the other hand, there have also been known methods of forming images according to the so-called dry process without liquid processing which processes a photosensitive material by peel developing or transfer developing.
In a method of forming images by use of such peel developing, as seen in, for example, Japanese Pat. Exam. Pub. No. 22901/1968, a material comprising a transparent support, a photopolymerizable composition layer and a second support is exposed and, then, the transparent support is peeled from the second support, so that a positive image is formed on one support and a negative image on the other support.
In a method which uses transfer developing, an image is formed by exposing an image through a color separation negative or mask on an image forming material comprising a support having thereon at least a photopolymerizable composition layer, and forming an image on an image receiving body by pressing and, if necessary, heating the exposed photopolymerizable composition layer with its surface in contact with the image receiving body and thereby transferring the unexposed portion to the image-receiving body, as described in U.S. Pat. Nos. 3,060,023, 3,060,024, 3,060,025, Japanese Pat. O.P.I. Pub. No. 147154/1988. These methods of forming images according to the dry processing are advantageous over the method using a liquid developer in environment pollution control, safety maintenance and apparatus downsizing.
With the progress of image processing technique, light source and image forming technique in recent years, there has come to be demanded a photosensitive material which is sensitive to light of longer wavelength. However, in the photosensitive compositions utilized in the foregoing methods of forming images, there have been used photopolymerization initiators, such as aromatic ketones including benzophenone, thioxanthone, quinone, thioacridone, and benzoin, benzil, benzylketal, which are sensitive in the ultraviolet wavelength region.
Various attempts have so far been made to make the photosensitive materials sensitive to a wavelength longer than that of visible light, as seen, for example, in Japanese Pat. O.P.I. Pub. Nos. 2528/1972, 155292/1979, 84183/1973, 151024/1979, 88005/1985, 40302/1983, 56403/1984, 189340/1984, 69/1990 and 189548/1990.
These photosensitive compositions are sensitive to visible light, but their sensitivity is not necessarily high enough to use practically. This necessitates a long exposing time and thereby impairs the efficiency of image formation by laser beam scanning exposure. In addition, these are not sensitive to a near infrared wavelength region corresponding to an oscillation wavelength of semiconductor lasers; therefore, semiconductor lasers which are the smallest and cheapest laser beam sources cannot be used.
Japanese Pat. O.P.I. Pub. No. 143044/1987 discloses a composition containing a novel photopolymerization initiator of cationic dye borate complex salt type, and a method of forming images by use of the initiator as well. However, the above patent specification does not disclose any initiator having a photosensitivity in the wavelength region of 780 nm or more which a semiconductor laser has; moreover, the method of forming images disclosed therein has a disadvantage of requiring complicated components such as micro-capsules.
Further, Japanese Pat. O.P.I. Pub. No. 4804/1990 discloses a photosensitive material, which is a combination of a cationic dye and a borate compound, and has a photosensitivity in the wavelength region of 600 to 900 nm to function as a photopolymerization initiator sensitive to a semiconductor laser. So far, however, there has not been known any method of forming color images by means of a simple dry process using a semiconductor laser and a photopolymerizable composition.